This was being reported by the Polish ambassador in the late 30's -- establishment historians argue that the mass-killings didn't begin until around early 1942.
The relentless anti-NS media campaign began early. In 1933, for example:
German Jewish groups, such as the Central Verein led by Stephen Wise, protested that Jewish-owned media were misrepresenting what was happening in Germany
An anti-Nazi rally was held in Madison Square Garden by the American Jewish Congress led by Stephen Wise where 40,000 attended
An economic and financial "Holy War" was declared against Germany, in an attempt to destroy its export trade.
None of this is a "secret", and it's not a post-war fabrication by the imaginary "neo-nazi" fifth column that hasbara love to harp on about this. What's happened is that this area of the history has disappeared down the memory hole, and people who bring it up are attacked as "anti-semitic."
"I emphasised that the defeat of Germany and Japan and their elimination from world trade would give Britain a tremendous opportunity to swell her foreign commerce in both volume and profit." - 'Baruch, The Public Years', by Bernard M. Baruch, 1960, p.347
"War in Europe in 1934 was inevitable." - H. Morgenthau, Secretary of the U.S. Treasury, Hearst Press, September, 1933
"There is only one power which really counts. The power of political pressure. We Jews are the most powerful people on earth, because we have this power, and we know how to apply it." - Vladimir Jabotinsky, Jewish Daily Bulletin, July 27, 1935.